The monster summoning glyph is one of the trickiest glyphs in the game to use effectively. If you try to utilize it in a naive fashion, it's largely useless. This is primarily because equal-level monsters are by far the worst opponents you could possibly be fighting, so you virtually never want to summon one. They grant no bonus experience, but are strong enough to deplete your health and mana. In terms of how much experience reward you get for how much effort you expend, this is a horrible deal and you should constantly be striving to avoid fighting these monsters, either fighting weaker monsters that you can kill effortlessly or stronger monsters that are worth bonus experience.
Even the slowing effect of this glyph is difficult to use. Most equal-level monsters are too strong to kill in a single blow, and slow will expire as soon as you strike them. This means that you usually cannot capitalize on the +1 experience bonus immediately after the monster is summoned unless you are strong enough to kill it in a single attack. Any character strong enough to do this should be capable of defeating considerably more powerful monsters and earning large amounts of bonus experience for doing so.
However, there is an unintuitive trick to make WONAFYT work effectively. To use this glyph to its full potential, go to an explored and largely empty part of the dungeon to summon monsters, then leave them there without attacking. This will corral weaker monsters that might otherwise be blocking hallways and place them out of your way. Since slow never expires, you can continue exploring and leveling up and return much later to kill them and gain the +1 XP bonus, allowing you to strategically level up at the most opportune time. In many ways, WONAFYT functions as a less expensive but much less controllable and practical version of WEYTWUT.
This glyph is only useful during early exploration. During later exploration its mana cost is rarely worthwhile, and as a higher-level character it's unlikely that you will ever be able to slay the monsters it summons in a single blow in order to take advantage of its slow effect (unless you are an Assassin). As a result, if you've already explored more than half the dungeon you should ignore this glyph completely and consign it for conversion.